Potential participants in the study will have been referred to the radiology section for a CTPA for a suspected pulmonary embolus. The radiologist will review the patient's record in CPRS, as is done routinely in this situation. Patients with class 3 or 4 CHF, supraventricular tachycardia, Serum Creatinine >1.5 mg/dl (>1.3 mg/dl female), history of contrast allergy, or unable to give informed consent will be excluded from both arms of the study. Patients with serum creatinine >1.28 mg/dl require referring physician approval. If the participant appears to qualify for the study then the study physician will contact the referring physician and inform him/her of the study. If the referring physician is agreeable then either the radiologist or the referring physician will approach the patient, either at the bedside or in the radiology clinic to determine if the patient is interested in participating in the study. If so, a study radiologist will begin the informed consent process. After informed consent is obtained the subject will be computer randomized to either the new low-dose or full-dose technique protocols. A total of 220 studies (110 per group) will be performed.